1. Field
The present invention relates generally to wireless communications. More specifically, the present invention relates to recovering information bits from code symbols and pilot symbols transmitted over diversity channels.
2. Background
Various over-the-air interfaces have been developed for wireless communications including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Communication systems configured in accordance with the IS-95 standard employ CDMA techniques to provide efficient and robust service. Exemplary communication systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
In any communication system employing coherent demodulation, a known pilot signal is transmitted along with a data-bearing signal so that a receiver can coherently demodulate the data-bearing signal and recover the transmitted data. Typical coherent receivers extract information about the wireless channel's amplitude and phase characteristics from the received pilot signal, and then use this extracted information for coherent demodulation of the data-bearing signal. Although information relevant to the channel characteristics is also present in the received data-bearing signal, it is not presently known in the art how a receiver can exploit this information.
The embodiments described herein exploit channel information in both the received pilot and data-bearing signals, and so achieve improved demodulation performance, as measured by gains in signal-to-noise ratio (SNR). Improvements in demodulation performance are clearly desirable because such improvements translate directly to increased robustness and indirectly to increased system coverage and capacity. In one embodiment, pilot and code symbols are used to determine optimized diversity combiner weights, which are then used by the receiver to determine the transmitted code symbols. In another embodiment, the optimized diversity combiner weights are used by the receiver to determine likelihood ratios for the received code symbols, which can then be used for soft-decision decoding of the transmitted data bits.